The present invention relates to a fast curing vulcanizable multi-part elastomer composition particularly suitable for injection molding. The invention also provides a process for blending, injection molding and curing of an elastomer composition to form an article.
In the rubber industry an elastomer composition is also commonly referred to as a “rubber compound”. A rubber compound is a blend of at least one elastomeric polymer (also referred to as “elastomer” or “rubber”) with a curing agent (or a combination of “curatives” as defined below), optionally with one or more filler, and additives such as antioxidants, antiozonants, lubricants, process aids, activators, oils, plasticizers, etc. A rubber compound typically is produced by blending the elastomer with the fillers and additives by mechanical means, usually an internal mixer or an open mill. To the resulting blend, also known as the “master batch”, a curing agent (or vulcanizing agent) and a cure rate accelerator are added. This combination of the vulcanizing agent and the accelerator is commonly referred to as the “curatives.” During conventional rubber processing, the “master batch” is combined with the “curatives” to form a homogenous mixture, known as the “finished compound”, which is ready to be formed and vulcanized into a finished article by an end user of the rubber compound. As mentioned above, the blending of the masterbatch and the curatives is typically carried out in an internal mixer or on an open mill. Since the elastomers used to produce rubber compounds typically have high molecular weights, the mixing of these materials generates considerable heat due to shear force. This heat may cause premature vulcanization or “scorch” since these rubber compounds, known as “accelerated compounds” at this stage, contain the cure rate accelerators as well as the vulcanization agent. Therefore, the mixing equipment is usually cooled to prevent premature vulcanization.
In a typical injection press operation a single rubber compound is fed, in the form of a single strip or strand or multiple strips or strands of the same compound, into the feed zone which contains a transport mechanism. This transport mechanism has a single purpose which is to move the compound into the injection holding chamber while warming up the compound, but not up to molding temperatures. The compound fills the injection holding chamber which is typically at a slightly higher temperature than the transport part of the injection press. The compound remains in the injection holding chamber until a portion of the compound is injected into the mold. After the mold is opened, the molded article is removed and the empty mold is closed, another portion of the rubber compound is injected into the mold.
In injection molding of rubber compounds, premature vulcanization or scorch prior to completion of the formation of the rubber compound into the desired article by molding causes defects in the properties of the molded product. In an ideal system for injection molding the rubber compound could be warmed up enough to have maximum flow without starting the cure reaction. This compound would be injected into the cavity of a mold which is set at a curing temperature. The amount of time available for the rubber compound to fill out the mold before vulcanization takes place is the “flow time”, which may be represented by the time measured in minutes for the viscosity of a rubber compound to increase by a certain number of units when measured by a Mooney shearing disk viscometer at a constant temperature, typically from 100° C. to 125° C. In practice, the flow time may be measured by the time for a rubber compound to fill out the cavity of a mold under conditions of constant temperature and pressure. The time elapsed for the Mooney viscosity to increase by 5 units is the Mooney t5 time, which may be used as a measure of the induction time for vulcanization or the scorch time of the rubber compound.
The curing properties of a rubber compound are traditionally measured by monitoring its torque using an Oscillating Disk Rheometer (ODR) at a curing temperature in the range of 150˜200° C., which is higher than the temperature at which the Mooney viscosity is measured. A rubber sample at room temperature (known as “cold rubber”) is placed in the instrument and the torque is measured as a function of time. The cold rubber offers resistance to the oscillating disk, which results in a spike in torque. This spike is referred to as the initial torque. As the sample warms up to curing temperature the sample becomes softer and the torque decreases. Then the sample begins to cure, which is shown by an increase in torque. A measure of the induction time for vulcanization is ts2, which is the time for the torque to increase by 2 units. As vulcanization proceeds the torque continues to increase until it reaches a maximum value. This maximum value is referred to as MH (maximum torque). The time to reach 90% of this increase in torque is referred to as t′90. A greater t′90 value means a longer cure time for the rubber compound. An ideal rubber compound would exhibit a Mooney t5 of at least 5 minutes for maximum scorch safety, a ts2 of between 0.7 and 1.2 minutes for ideal flow, with a short t′90 for fast cycle time. As a general rule of thumb, the longer the induction time (as measured by ts2) a compound has means a longer cure time (as measured by t′90.) Compounds with ts2 values of less than 0.6 minutes and Mooney t5 values of less than 3 minutes are considered “scorchy” and difficult to process. Such a compound would undergo rapid cross-linking before the compound has completely filled the cavity of a mold. Compounds with long ts2 values greater than 1.5 minutes would have a difficult time curing in 1 minute, which is an economically desirable cure time.
To minimize scorch or premature curing, rubber compounds are formulated to have a slow cure rate either through the use of chemical cure rate retarders or through judicious selection of accelerators.
At the completion of the molding process the obtained article must be allowed to undergo complete curing by holding the molded article for a period of time under curing conditions, such as a sufficiently elevated temperature for a sufficient amount of time.
Not all compounds can be economically vulcanized in an injection press; therefore a secondary vulcanizing step (or post-cure) may be required. This “post-cure” may be carried out by holding the molded article inside the mold or press for an extended period of time, but doing so has a drastic, negative economic impact on production rates. It is more economical to remove the molded article prior to complete vulcanization, and subject it to an additional curing step outside the mold or press. Even though it is more economical than post-curing inside the press, this additional curing step outside the press also adds significantly to the cost and reduces the efficiency of the molding process.
An ideal rubber compound would exhibit a Mooney t5 of at least five minutes for maximum scorch safety and ideal flow, with a short t′90 for fast cycle time. As a rule of thumb, the longer the scorch time (as measured by Mooney t5) a compound exhibits usually translates into a longer cure time (as measured by t′90.) Compounds with Mooney t5 values of less than 3 minutes are considered “scorchy” and difficult to process. Compounds with Mooney t5 values of 3 minutes or more are processed more easily, but with the disadvantage of requiring a longer cure time.
A rubber compound having the ideal flow and curing properties described above has not been realized. The present invention resulted from the inventors' efforts to develop a rubber compound that approximates the properties of an ideal rubber compound, particularly for use in injection molding, and to develop an improved process for blending, injection molding and curing of a rubber compound.